Methylation of the CpG dinucleotide occurs in a characteristic pattern in the DNA of higher order eukaryotic tissues. Recent data demonstrate that this pattern is altered in primary tumors, particularly leukemias, and in cell lines. These cells exhibit hypermethylation in characteristically unmethylated CpG islands in the calcitonin gene on chromosome 11p and at other loci. Likewise, expression of DNA methyltransferase, which catalyzes CpG methylation, is increased in these cells. CpG methylation can inhibit transcription at numerous tissue specific genes by inhibiting sequence- specific protein binding and/or altering chromatin structure. Thus, regional hypermethylation may disrupt the functional specialization and loss of proliferative potential characteristic of differentiating hematopoietic cells. Elucidation of the pattern of, and mechanisms governing, methylation in normal and abnormal hematopoiesis is therefore necessary to define the role of regional hypermethylation in the process of cellular immortalization and leukemic transformation. The experiments proposed herein will define overall and gene-specific methylation patterns in purified populations of normal, stage-specific, myelomonocytic cells and undifferentiated progenitors compared to those of AML cell lines and primary myeloid leukemias. The expression of DNA methyltransferase will be mapped as a function of normal and leukemic differentiation. The existence of specific DNA sequences directing de novo methylation, perhaps operative in regional hypermethylation, will be investigated. Finally, to begin to further investigate the mechanisms governing methylation in hematopoiesis, a genomic clone encoding the promoter of the DNA methyltransferase gene will be isolated and analyzed.